1. Field of the Invention
This invention relates to a pre-chamber type engine having main chambers, pre-chambers and fuel injection nozzles the injection ports of which are opened in the pre-chambers.
2. Description of the Prior Art
A swirl chamber type engine having swirl chambers has heretofore been developed for the purpose of improving the combustion in the engine. Such a swirl chamber type engine has swirl chambers formed in a cylinder head, communication ports via which the swirl chambers and main chambers formed in cylinders communicate with each other, and fuel injection nozzles provided in the swirl chambers, and the swirl currents flowing into the swirl chambers via the communication ports and a fuel injected into the swirl chambers form a gaseous mixture.
The swirl chamber type engines of this kind include as an example thereof a pre-chamber type structure disclosed in Japanese Patent Laid-open No. 12613/1990. In this pre-chamber type structure, the pre-chamber walls defining pre-chambers communicating with main combustion chambers formed in the cylinders via injection ports are formed out of an insulating material, and film members consisting of a ceramic material are provided on the inner surfaces of the pre-chamber walls. In this pre-chamber type structure, metal protective cylinders are fitted firmly over the outer circumferential surfaces of the pre-chamber walls. The injection ports allowing the main chambers to communicate with the pre-chambers are formed in a cylinder head incliningly with respect to the pre-chambers so as to promote the formation of swirl currents.
In general, the combustion temperature in a pre-chamber is high, so that it is effective to burn a gaseous mixture at a low air-fuel ratio for the purpose of reducing the generation of NOx. In order to burn a gaseous mixture at a low air-fuel ratio when the combustion temperature is high, an engine of a pre-chamber type structure is effectively used. In order to increase the combustion speed in a pre-chamber type combustion chamber in a pre-chamber type combustion chamber-carrying engine to a level substantially equal to that of the combustion speed in a direct injection type combustion chamber, it is necessary that the cross-sectional area of a communication port via which a pre-chamber and a main chamber communicate with each other be increased. However, when the cross-sectional area of the communication port is increased, the injection speed of a fuel from the pre-chamber into the main chamber decreases, and the combustion in the main chamber is not carried out satisfactorily.
In a conventional swirl chamber type engine, the rate of generation of NOx and smoke increases unless the direction in which a swirl current formed in a swirl chamber flows, the direction in which an atomized fuel is ejected from a fuel injection nozzle and the timing at which the fuel is thus ejected from the same nozzle all match. In a swirl chamber type engine, a communication port via which the pre-chamber and main chamber communicate with each other is small, so that a throttle loss due to the communication port occurs, which causes an engine output to decrease. In general, a communication port via which a main chamber and pre-chamber communicate with each other is provided in either the central portion of a cylinder or one spot of the outer circumferential portion thereof. This causes the distance over which an ejection current travels to increase, the sufficient mixing of the ejection current with the air in the main chamber to be prevented, HC and smoke to occur, the combustion time to be prolonged and the performance of the engine to lower. Since the communication port is restricted, the suction air entering the main chamber via the suction port and flowing into the pre-chamber through the communication port causes a strong swirl current to occur therein, and, due to this air flow, the heat transfer rate of the inner surface of the pre-chamber increases to cause an increase in the radiation rate.
In a direct injection engine having a combustion chamber in a piston, the volume ratio of the piston combustion chamber, i.e. a ratio of the volume of the piston combustion chamber to that of the whole combustion chamber at the end of a compression stroke is generally 65-80%, and the volume of a space not included in this percentage is called useless volume, attempts having been made to reduce the useless volume to the lowest possible level.